Simple 5 gal extract brew with speciatly grains, English Light Ale. Starting gravity 1.042. Pitched around 74 F and had normal activity for 24 hours, but temp in fermenter was not coming down despite being in ~70 F ambient. I let the room temp fall back to ~64 F over night and the next morning my fermenter was down to ~66-67. It remained there for a week with little activity... racked to secondary at 1.018. Brew was quite cloudy at racking and smelled very... green? or strong at racking. It's been in the secondary almost a week at a constant 68 F, I haven't seen the airlock even change position, like nothing has occured. Got a good seal, its in a carboy with stopper... push on the stopper i can make the water move in the lock.

Did I somehow stop the yeast with too fast of a temperature change... or is there still some activity ongoing despite seeing no signs? Going to give it another week and take a reading I guess, would expect ~1.010 FG.

You didn't kill your yeast, but dropping the temp quickly by even a few degrees can cause your yeast to deactivate and flocculate out. Give the carboy a swirl to bring any settled out yeast back up into suspension - that can sometimes get them restarted. Wouldn't hurt to warm it back up by a couple degrees and give it a little more time. But I've had extract+specialty grain beers finish at 1.020 before, so it might actually be finished...

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Random Fact: Human Immunodeficiency Virus particles have approximately the same density of 1.170 wort. I know because I float them on a sucrose gradient in the lab to purify them for research.

If you weren't at final gravity expected...why did you rack? Time doesn't dictate racking or bottling. The beer being DONE according to a steady gravity at or very close to the expected gravity is what dictates when we do things.